Hydraulic pumps and motors and the fluid (oil, synthetic liquids or the like) used with them are commonly employed for transferring power from one location to another. A very common type of hydraulic circuit includes a hydraulic pump driven by a "prime mover" source of power such as an electric motor or an internal combustion engine. The pump draws oil from a tank and delivers such oil under pressure to one or more control valves. The control valve(s) direct pressurized oil to one or more hydraulic output mechanisms used to perform useful work. Exemplary output mechanisms include hydraulic cylinders (sometimes referred to as linear motors) and rotary hydraulic motors.
Hydraulic circuits can be (and are) used in a wide variety of applications and offer very good control of the output mechanism powered by such circuit. Because the circuit components, i.e., pumps, valves, motors and the like, can be connected to one another using flexible hoses, hydraulic circuits transfer power in situations where it is not possible (or at least not practical) to use straight, rigid mechanical drive lines for that purpose. And when compared to mechanical drive lines, hydraulic circuits offer superior "controllability."
Many types of hydraulically-powered machines involve hydraulic output mechanisms, e.g., hydraulic motors, the sizes and maximum speeds of which are known to the designer and do not change over the life of the machine. For example, a self-propelled agricultural combine for harvesting row crops and grain has a number of hydraulic mechanisms, the power requirements of which are known to the designer. The hydraulic motor(s), pump(s), valve(s) and the like are sized in anticipation of known mechanism load characteristics.
But design engineers do not always have the luxury of "predictability of load." Some hydraulic machines are specifically configured in anticipation of having any of a wide variety of equipment types powered by the hydraulic circuit on such machine. A good example is an agricultural tractor.
Such tractors are used to pull equipment such as a towed combine, a crop sprayer, a crop planter or a forage harvester, to name but a few. Each type of equipment has its own hydraulic mechanisms which will likely differ as to number and flow and pressure requirements from those of another equipment type.
And the precise make and model of the equipment which may be towed by a particular tractor is not known to the designer. Of course, purchasers of tractors are not required to inform the tractor seller of all of the types of towed equipment that may be used with such tractor. And in any event, such equipment may change over time. It is fair to say that in the foregoing examples, the tractor is the "power supply" for the towed equipment used therewith.
These facts present a challenge to the designer of the tractor hydraulic circuit who must anticipate the number and types of hydraulic mechanisms on the equipment being towed and powered by the tractor. And the flow requirements of such mechanisms vary widely.
A common practice when designing certain known hydraulic circuits is to select pumps which, in both number and maximum output flow of each, will meet the needs of the highest anticipated equipment flow requirement. As a result, such circuits have excess flow capacity for many types of less-demanding equipment--one or more hydraulic pumps may be utilized only a small percentage of the tractor operating time. To state it another way, the pumping capacity is selected for maximum load requirements and is under-utilized during "off-peak" operating periods.
The inclusion of such pumping capacity can be burdensome to both the tractor designer and the user. The designer must find locations on the tractor to mount pumps (which are driven by the tractor internal combustion engine) and must consider the added cost thereof when setting the tractor selling price. And the tractor user is required to maintain such pumps and keep them operating.
An approach to minimizing the installed pumping capacity on a machine involves what is known as "load sensing." U.S. Pat. No. 4,470,259 (Miller et al.) describes a closed-center load-sensing hydraulic system having a pressure-compensated, variable-displacement pump and two functions "prioritized" one to the other. The primary work circuit (e.g., steering system) is given priority in flow over a secondary work circuit. U.S. Pat. No. 4,470,260 (Miller et al.) shows a similar system which is open-center and uses a fixed-displacement pump. Both Miller et al. systems are said to alleviate steering wheel "kickback."
The system of the Miller et al. '259 patent is configured so that the vehicle steering system has priority and controls whether pump flow is directed only to the steering system, to both the steering and secondary work circuits or only to the secondary work circuits. The system of the Miller et al. '260 patent initially directs all fluid to the primary work circuit and then, depending upon rising load pressure in the primary circuit, to both the primary and secondary work circuits or to the secondary work circuit alone.
To put it in other words, the Miller et al. systems employ a single pump for powering two functions. The systems "favor" the primary function and shift pump flow to the secondary function only when the primary function is satisfied.
The system described in U.S. Pat. No. 5,289,680 (Obe et al.) has three hydraulic pumps. The first pump normally powers a working implement but its flow can be manually valved to join the flow of a second pump which powers what the patent calls an auxiliary actuator. U.S. Pat. No. 5,313,795 (Dunn) describes a tri-path pressure selector network that prioritizes the flow of a pump to a vehicle braking system. When the needs of such system are satisfied, pump flow is available for other functions.
While these prior art systems have been generally satisfactory for their intended purposes, they are less-than-ideally suited for applications where the nature of the load mechanisms to be powered by the system is not fully known. The system of the Obe et al. patent relies upon manual valve manipulation to shift the flow of one pump between two circuits, one of which is also "fed" by a second pump. It is difficult for an operator to know when a mechanism needs additional hydraulic flow (and when it does not) and in any event, manual manipulation often results in reduced machine efficiency.
The rationale underlying the systems of the Miller et al. patents is to always "favor" the primary work circuit over a secondary circuit rather than to direct pump flow to the primary circuit only during peak demand. This approach is clearly appropriate where the primary work circuit involves steering or braking but less than satisfactory for powering other implement mechanisms.
A new hydraulic circuit which minimizes the pumping capacity installed on a machine, which supplements main pump flow only during peak demand imposed by the hydraulic mechanisms being powered, which is automatic in operation and which takes advantage of the operating characteristics of a variable-delivery pump and the simplicity of a fixed-delivery pump would be an important advance in the art.